Why are these lakes salty? Rivers flow into them, but no rivers flow out. The lakes do not overflow, because some water turns into vapor and evaporates. Salts left behind salinate the water.

The Caspian Sea, bordered by Russia, Kazakhstan, Turkmenistan, Iran, and Azerbaijan, is the largest inland body of water in the world. About the size of California, it creates a habitat for plants and animals found nowhere else in the world. It also has enormous reserves of oil and natural gas.

This oil and gas production has led to an increase in pollution in the Caspian Sea region. Industrial and chemical wastes collect in the sea with no way to flow out. This is likely one cause of the dramatic decline in the numbers of sturgeon, a fish found nearly exclusively in the Caspian Sea. Another reason is the construction of dams and canals to help water nearby crops. The fish can't travel to reproduce.

Humans have also adversely affected the Aral Sea, which used to be the fourth largest lake in the world. Today, much of the sea is desert.

Located between Uzbekistan and Kazakhstan, the Aral Sea has lost almost 75 percent of its water since 1960, when the Soviet Union diverted water from inflowing rivers to irrigate cotton crops in Central Asia. The sea has dried up so much that it is now divided entirely into two parts, the Greater Sea and the Lesser Sea.

The desertification of the Aral has had many effects. Nearly 60,000 fishermen lost their livelihoods when the fish population declined. Villages that were once on the lake's shores are now miles from water. And people have become ill from breathing mineral- and pollution-laden dust blowing off the dry seabed.

Perhaps the most serious change is one in the region's weather. The vast expanse of the Aral used to absorb summer heat and keep winters mild. Now, hotter summers and longer, colder winters have shortened the growing season for crops.

Central Asian countries are trying to correct the environmental damage and restore the Aral Sea, which could dry up altogether by 2015, scientists warn.

The lowest point on Earth, the Dead Sea sits between Jordan and Israel in a valley 1,349 feet (411 meters) below sea level.

The water of the Dead Sea is about 25 percent salt—seven times saltier than the ocean—and so buoyant that most swimmers can't put their feet down.

Nothing except bacteria and small shrimp can grow in the sea, which is lined with salt formations. About half the lake's water evaporates in the hot, dry climate every year.